With the increasing traffic demand, cellular operators require more spectrums in addition to a licensed band. The Long Term Evolution (LTE) deployment in an unlicensed band (LTE-U), typically over 5 GHz, can be potentially implemented to offload mass traffic from the licensed carriers. A Study Item on LTE-U has been initiated in 3rd Generation Partner Project (3GPP) Rel-13 for standardization of this feature. The first stage of LTE-U would consist of utilizing the unlicensed band for supplementary downlink (SDL) and the uplink is sent using the licensed band. Since LTE-U may partially operate in the unlicensed band, it would have to co-exist with other network, particularly other LTE-U and Wi-Fi (e.g. 802.11ac).
The hidden terminal is a well-known problem in Wi-Fi. FIG. 1 illustrates a schematic view of hidden terminal interference between APs. FIG. 1 contains two Wi-Fi Access Points (AP): AP#1 and AP2. Terminal 1 is connected to AP1 whilst Terminal 2 is connected to AP2. AP1 cannot hear the transmission from AP2 but Terminal 2 can hear both AP#1 and AP2 transmission. Whilst AP2 is transmitting data to Terminal 2, AP#1 failing to sense AP2 transmission may transmit data to Terminal 1, which causes interference to Terminal 2.
Therefore, carrier sense multiple access with collision avoidance (CSMA/CA) is introduced in Wi-Fi to mitigate the hidden terminal problem. FIG. 2 illustrates a schematic view of CSMA/CA for avoiding hidden terminal interference between Wi-Fi APs. As illustrated in FIG. 2, AP2 transmits a “Request to Send” (RTS) to Terminal 2 prior to transmitting any actual data. Any terminals detecting the RTS would refrain from transmission. In response to the RTS, Terminal 2 would sense the channel and if it is free, it would response with a “Clear to Send” (CTS) back to AP#2. Similarly any terminal that can detect the CTS will refrain from transmitting. In FIG. 2, AP1 would detect the CTS and would not transmit data to Terminal 1. Upon receiving the CTS from Terminal 2, AP2 would start its data transmission to Terminal 2. In this way the Terminal 2 would avoid being interfered by AP1.
The CSMA/CA mechanism can also be adopted in LTE-U for LTE-U/LTE-U and LTE-U/Wi-Fi coexistence. However, it should be appreciated that LTE-U and Wi-Fi have different methods of operation. That is, in Wi-Fi transmission from a terminal is based on contention, i.e. a terminal would seize the channel when it is available. In contrast, in LTE-U, resources are managed and scheduled by the eNB. Since one possible mode of operation of LTE-U is for downlink transmission only, the CTS from the (LTE-U) UE to the eNB is transmitted in the licensed band. How to schedule the resource for CTS transmission in licensed uplink band is an essential problem to be solved.